Soil Quality Management and Restoration Recently, requirements within the State of Iowa’s NPDES General Permit #2 for construction sites were amended. These changes removed a requirement to restore four inches of topsoil across disturbed open spaces. The permit now requires that topsoil be preserved on site where feasible, but does not specify where and how that topsoil is to be placed or preserved. During the discussions leading up to these changes, many concerns were raised by development and real estate interests about the cost and timing of restoring topsoil, especially on finished lawn spaces within single-family land developments. Conceivably, the changes in permit language allow topsoil to be preserved within berms or other confined areas and may not be placed uniformly across the landscape. This means that many open spaces may lack the healthy soil material needed to support the growth of lawns and landscaping. Should this occur, the soil will have limited ability to absorb runoff during rainfall events (runoff volumes may be increased by more than 80% during the most commonly occurring storm events) (1). Higher levels of watering and fertilization will be necessary to support desired plant materials. All of these factors have the potential to increase stormwater runoff volume and pollutant loads.
For this reason, it is recommended that communities implement local ordinances to protect or restore healthy soils in open space areas within new development sites. The Iowa Stormwater Management Manual has an entire chapter devoted to the topic of maintaining and restoring healthy soil profiles. Options include limiting the footprint of land disturbance, topsoil stripping/replacing or using soil amendments like compost and sand to rebuild a healthy surface topsoil layer.
To fully realize the benefits of soil quality restoration, the methods within ISWMM manual list various ways to maintain or create eight inches of a healthy soil profile across the surface. Requirements to achieve this standard can be incorporated into other ordinances, or implemented as a stand-alone ordinance.
Such requirements should include the following elements:
- All construction sites which are subject to local grading permit or State NPDES permit requirements should develop and maintain a Soil Management Plan (SMP) which becomes a part of the SWPPP document when one is created for a given site.
- The SMP shall review soils information from county maps, geotechnical studies or other sources to identify where higher quality soils may exist. When possible, the organic content of onsite topsoil material should be determined by testing.
- To the extent possible, site improvements should be oriented to minimize disturbance of high quality soils. Site grading should be planned to avoid compacting, filling or tilling under the drip line of trees which are identified as being intended to be preserved through construction.
- Identify where topsoil will be stripped, stockpiled and replaced. The quantity of stockpiled material should be estimated.
- Where grading is necessary, show the location and type of method of Soil Quality Restoration (SQR) to be applied (reference ISWMM chapter to see the available methods and how they are achieved).
- In some locations, it is possible to use SQR techniques to partially or totally address the Water Quality Volume. If this is proposed, identify locations where SQR techniques are intended to be used to meet such requirements. Include relevant calculations to demonstrate compliance with requirements listed in the ISWMM manual within a stormwater management report submitted to the local jurisdiction for review.
- If SQR techniques are not proposed, or not applied, appropriate adjustments to runoff coefficients and curve numbers within stormwater design calculations should be made to account for the effects of soil compaction and poor establishment of vegetation. The ISWMM manual includes recommendations on how to account for these effects.
Application It is recommended that ordinance and policies be implemented that would apply these standards to all sites requiring either a local grading permit or authorization under the State of Iowa’s NPDES General Permit No.2 (construction sites or common plans of development which will disturb at least one acre).
Historic topsoil depth and organic matter levels have been reduced in agricultural areas. The remaining topsoil is often stripped off or compacted during grading and construction of new land developments.
The Iowa Stormwater Management Manual contains a section on Soil Management and Restoration. It designates eight different methods that can be used to protect or restore a healthy topsoil layer during the construction process. Designers can use this information to develop a Soil Management Plan, which outlines how developers or contractors can use one or more of these eight methods to leave lawn and landscaping areas with adequate topsoil to support vegetation and reduce stormwater runoff.